This application claims priority benefits under 35 USC xc2xa7 119 on the basis of Japanese Patent Application No. 2000-99539, the disclosure thereof being incorporated herein by reference.
The present invention relates to a throwaway tip for drilling. The invention further relates to a drill holder for holding such a throwaway tip at a front end thereof.
Drills for drilling a workpiece such as of a metal include: solid type drills which have a unitary drill structure; and throwaway-tipped drills which have a drill holder, and a throwaway tip having a cutting edge and adapted to be removably attached to a front end of the drill holder.
The throwaway-tipped drills are classified into a double tip type and a single tip type.
A throwaway-tipped drill of the double tip type has two tips including an inner cutting tip which has an inner cutting edge for cutting a center area of a hole being drilled, and an outer cutting tip which has an outer cutting edge for cutting a circumference area of the hole, the two tips being attached to a front end of a drill holder (Japanese Laid-Open Patent Publication JP 10-29108A(1998)).
The construction of the double tip type is suitable for a larger-size drill having a greater drilling diameter. However, it is difficult to apply this construction to a smaller-size drill having a drilling diameter of smaller than xcfx8610 for the following reasons.
It is difficult to provide a space sufficient to accommodate the two tips attached to the front end of the holder.
Further, it is difficult to provide a space for smooth ejection of chips generated by cutting a workpiece with the inner cutting edge and the outer cutting edge of the tip.
Since screws for fixing the tips to the holder are very small in size, a sufficient fixing strength cannot be ensured.
Therefore, the smaller-size drill mainly employs the construction of the single tip type.
The throwaway-tipped drill of the single tip type generally employs a tip having a similar tip shape to the conventional solid type drill. More specifically, a tip 91 is employed which has a pair of long cutting ridges 91b, 91b each extending from a tip point 91a positioned on a rotation axis of the drill across the entire radius of a hole being drilled as shown in FIGS. 8A to 8C (Japanese Laid-Open Patent Publication JP 10-328918A(1998)).
However, the cutting ridges 91b, 91b of the tip 91 each have a great length, so that the tip 91 is subjected to a great cutting resistance during drilling. Therefore, it is necessary to firmly fix the single tip 91 to a front end of a holder 93 with two screws 92, 92. Further, the tip 91 needs to have a greater thickness. This makes it difficult to provide a space for ejecting chips from the hole.
The inventor of the present invention previously developed a throwaway tip of the single tip type having a similar front portion shape to that of the throwaway-tipped drill of the double tip type (Japanese Laid-Open Patent Publication JP 11-188518A(1999)). This throwaway tip has an inner cutting edge and an outer cutting edge which are respectively modeled after the inner cutting edge of the inner cutting tip and the outer cutting edge of the outer cutting tip of the throwaway-tipped drill of the double tip type and formed integrally on the single tip.
The inner cutting edge of the throwaway tip is merely required to have a length sufficient to cut a center area of a hole being drilled. Similarly, the outer cutting edge is merely required to have a length sufficient to cut a circumference area of the hole. Therefore, the lengths of the inner cutting edge and the outer cutting edge can be reduced as compared with the cutting ridges of the conventional single tip type drill which each extend across the entire radius of the hole, so that the cutting resistance exerted on the tip during drilling can be reduced. As a result, the thicknesses of the inner cutting edge and the outer cutting edge of the tip can be reduced, and the tip can be fixed to a holder with a single screw. Further, a sufficient space can be provided for ejecting chips from the hole.
The inventor has recently conducted studies on the throwaway tip and the drill holder to be employed in combination with the throwaway tip. As a result, the inventors have found that the constructions described in the aforesaid publications need further improvement in the function thereof, particularly, for smoothly ejecting the chips from the hole.
It is an object of the present invention to provide a novel throwaway tip which allows for easier chip ejection than the prior art. It is another object of the invention to provide a novel drill holder for a throwaway tip which allows for easier chip ejection than the prior art.
In accordance with an inventive aspect of claim 1, there is provided a throwaway tip for drilling a hole, which comprises: an inner cutting edge provided at a front end of a drill direction of the tip for cutting a center area of the hole; an outer cutting edge provided at the front end of the tip for cutting a circumference area of the hole; a first rake face provided on one side of the tip; a first bearing face provided on the other side of the tip in a back-to-back relation with the first rake face; a first front flank provided on the front end of the tip as intersecting the first rake face and the first bearing face; the inner cutting edge being defined as a ridge by an intersection between the first rake face and the first front flank and extending as crossing the drilling direction; a second rake face provided on the other side of the tip; a second bearing face provided on the one side of the tip in a back-to-back relation with the second rake face; and a second front flank provided on the front end as intersecting the second rake face and the second bearing face; the outer cutting edge being defined as a ridge by an intersection between the second rake face and the second front flank and extending as crossing the drilling direction; the first rake face being located at a lower level than the second bearing face on one side of the tip, and connected to the second bearing face by a first curved surface for guiding chips generated by cutting with the inner cutting edge.
Conventionally, the inventor has designed a tip so that a greater chip ejection space can be provided on the side of the inner cutting edge than on the side of the outer cutting edge for ejecting chips generated on the side of the inner cutting edge as smoothly as possible. This is because chips generated on the side of the inner cutting edge and on the side of the outer cutting edge are different in shape and, hence, are ejected through different chip ejection mechanisms.
For example, a chip K generated when a portion of a workpiece around the center of a hole being drilled along the rotation axis Pv of a drill is cut with the inner cutting edge 11 is in a three-dimensional helical shape such that cones, of which apex corresponds to a center of the hole, are stacked one on another as shown in FIG. 4A. This is because a rate at which the workpiece is cut with the inner cutting edge is zero at the center of the hole and increases as a radial distance from the center increases. In order to smoothly eject the chip K of such a complicated three-dimensional shape from the hole, it is necessary to keep the chip continuous with least discontinuity. The continuous chip K without discontinuity gradually grows in a direction indicated by a solid line arrow in the figure as the drilling proceeds, and is automatically and continuously carried away from the hole through a flute of the holder.
On the contrary, a chip generated when a relatively small portion of the workpiece which corresponds to a circumference area of the hole is cut with the outer cutting edge is in the shape of a strip having a given width or segments of the strip. This is because the outer cutting edge does not produce so great a difference in cutting rate between an axial side and a circumferential side in a circular locus thereof during rotation as the inner cutting edge does. The chips generated on the side of the outer cutting edge are more easily ejected than the chips generated on the side of the inner cutting edge.
Therefore, the inventor considered that, in order to smoothly eject the chips of the complicated three-dimensional shape on the side of the inner cutting edge, the chip ejection space provided on the side of the inner cutting edge desirably has a greater size than the space provided on the side of the outer cutting edge for the ejection of the fine chips.
However, studies thereafter conducted by the inventor revealed that the provision of the greater space alone produces a limited effect on the smooth chip ejection on the side of the inner cutting edge. This indicated that there are limitations on further increase in the rotation speed of the drill and, hence, on increase in the drilling rate.
Therefore, the inventor contemplated a design such that a middle portion of the tip serving for the chip ejection on the side of the inner cutting edge is configured to be suitable for guiding and ejecting the chips of the complicated three-dimensional shape.
As a result, the inventors have come up with an idea that a step gap between the first rake face 13a and the second bearing face 1c on the side of the inner cutting edge 11 is spanned with the first curved surface 15 for guiding the chips as set forth in claim 1 (FIG. 1A).
With the arrangement according to claim 1, the chips of the complicated three-dimensional shape can more smoothly and continuously be ejected from the hole through the flute of the holder than in the prior art while being guided by the first curved surface. This makes it possible to further increase the rotation speed of the drill and, hence, to increase the drilling rate.
In accordance with an inventive aspect of claim 2, the throwaway tip according to claim 1 is characterized in that the first curved surface has a curvature radius which is 7 to 15% of a drilling diameter of the tip.
If the curvature radius of the first curved surface is smaller than 7%, the curvature radius is so small that the chips generated by cutting the workpiece with the inner cutting edge are liable to be broken on the first curved surface. Hence, there is a possibility that the effect of the provision of the first curved surface cannot satisfactorily be provided for continuously and smoothly guiding and ejecting the chips generated on the side of the inner cutting edge without discontinuity. If the curvature radius is greater than 15%, the chip ejection space provided by locating the first rake face at the lower level than the second bearing face is narrowed. Hence, there is a possibility that the effect of the provision of the first curved surface cannot satisfactorily be provided for continuously and smoothly guiding and ejecting the chips generated on the side of the inner cutting edge.
In accordance with an inventive aspect of claim 3, the throwaway tip according to claim 1 is characterized in that the second rake face is located at a lower level than the first bearing face on the other side of the tip, and connected to the first bearing face by a step face including a second curved surface.
The arrangement according to claim 3 makes it possible to relieve a stress which may otherwise be concentrated on a corner edge of the step between the second rake face and the first bearing face on the side of the outer cutting edge which is subjected to a greater cutting resistance due to a greater circumferential speed than the inner cutting edge. Thus, the throwaway tip has an improved strength.
In accordance with an inventive aspect of claim 4, the throwaway tip according to claim 3 is characterized in that the first curved surface has a greater curvature radius than the second curved surface.
The arrangement according to claim 4 further facilitates the chip ejection on the side of the inner cutting edge. In addition, a greater space can be provided on the side of the outer cutting edge for the chip ejection by the step between the second rake face and the first bearing face, thereby facilitating the chip ejection on the side of the outer cutting edge.
In accordance with an inventive aspect of claim 5, the throwaway tip according to claim 3 is characterized in that the second curved surface has a curvature radius which is 5 to 7% of the drilling diameter of the tip.
If the curvature radius of the second curved surface is smaller than 5%, there is a possibility of failing to satisfactorily provide the effect of relieving the stress concentrated on the corner edge of the step for the improvement of the tip strength. If the curvature radius is greater than 7%, the chip ejection space provided by locating the second rake face at the lower level than the first bearing face is narrowed. Hence, there is a possibility of interfering with the chip ejection on the side of the outer cutting edge.
In accordance with an inventive aspect of claim 6, the throwaway tip according to claim 1 is characterized in that the first rake face has a first breaker concavity.
The arrangement according to claim 6 makes it possible to neatly curl the chips generated by cutting the workpiece with the inner cutting edge along a bottom face of the first breaker concavity. This allows the chips of the complicated shape generated on the side of the inner cutting edge to smoothly grow without discontinuity, thereby further facilitating the chip ejection.
In accordance with an inventive aspect of claim 7, the throwaway tip according to claim 6 is characterized in that the first curved surface is raised from the first breaker concavity forwardly with respect to a chip ejecting direction.
With the arrangement according to claim 7, the combination of the first breaker concavity and the first curved surface allows the chips generated by cutting the workpiece with the inner cutting edge to be neatly curled along the bottom face of the first breaker concavity and to be smoothly guided and ejected along the first curved surface. This further facilitates the chip ejection.
In accordance with an inventive aspect of claim 8, the throwaway tip according to claim 6 is characterized in that the first breaker concavity has a width, as measured in the drilling direction, which is 15 to 25% of the drilling diameter of the tip.
If the width of the first breaker concavity is smaller than 15%, the chips generated by cutting the workpiece with the inner cutting edge are liable to be broken when the chips are curled along the bottom face of the first breaker concavity. On the contrary, if the width is greater than 25%, the curled chips each have an excessively great curvature radius. In either of the cases, there is a possibility that the chips cannot be neatly curled to be smoothly ejected.
In accordance with an inventive aspect of claim 9, the throwaway tip according to claim 6 is characterized in that the second rake face has a second breaker concavity, and the first breaker concavity has a greater width as measured in the drilling direction than the second breaker concavity.
The arrangement according to claim 9 can further enhance the effect provided by the arrangement according to claim 6. In addition, the strip-shaped chips generated by cutting the workpiece with the outer cutting edge are bent at acute angles along a bottom face of the second breaker concavity so as to be finely broken for easier ejection thereof.
In accordance with an inventive aspect of claim 1, the throwaway tip according to claim 9 is characterized in that the second breaker concavity has a width, as measured in the drilling direction, which is 5 to 15% of the drilling diameter of the tip.
If the width of the second breaker concavity is smaller than 5% or greater than 15%, there is a possibility that the effect provided by the arrangement according to claim 9 cannot be ensured. That is, the chips generated by cutting the workpiece with the outer cutting edge cannot be finely broken by the second breaker concavity. This may make the chip ejection less easy.
In accordance with an inventive aspect of claim 11, there is provided a drill holder adapted to hold a throwaway tip at a front end thereof for drilling a hole, the drill holder comprising: a pair of holder pieces provided at the front end of the holder for holding the tip therebetween; a pocket for receiving the tip inserted therein between the holder pieces; and a pair of helical flutes provided on an outer circumferential surface of the holder for ejecting chips generated by cutting a workpiece with inner and outer cutting edges of the tip inserted in the pocket from the hole; wherein a connection surface extending from one of the helical flutes to reach a first rake face of the tip on the side of the inner cutting edge is inclined along an inclination of the one helical flute.
The arrangement according to claim 11 makes it possible to smoothly eject the chips of the complicated three-dimensional shape generated on the side of the inner cutting edge from the first rake face of the tip through the inclined connection surface into the flute. This further facilitates the chip ejection.